Repeated nonlethal doses of a widely used pesticide can change the outcomes of fights between ant species.

Repetitive exposure to traces of nicotine-related pesticides called neonicotinoids may alter who wins when invasive species clash with natives, says entomologist Rafael Barbieri of Victoria University of Wellington in New Zealand.

In small battles that he and his colleagues staged in the lab, New Zealand’s native southern ant could exterminate an equal force of invasive Argentine ants in about 16 hours, if the invaders had been repeatedly exposed to neonicotinoids. Yet if the natives were contaminated too, they failed to wipe out opponents as quickly. Some of the invaders were still alive at the end of the experiment, after 32 hours, Barbieri and his colleagues report October 23 in the Proceedings of the Royal Society B.

The neonicotinoid pesticides get their killing power by sabotaging insect nerve-signaling systems. Nonlethal doses, however, may cause more subtle damage, a string of recent research papers suggests. Chronic exposure may impair foraging and slow colony growth in honeybees and bumblebees (SN: 5/5/12, p. 8). As far as Barbieri knows, however, his paper is the first to test neonicotinoids’ effects on the power balance between species.

Barbieri and helpers dug up ants in the wild and created 10 test colonies of southern ants (Monomorium antarcticum) and 10 of Argentine ants (Linepithema humile), all of equal sizes. To mimic inadvertent exposure to pesticide, the researchers offered all colonies a honey solution contaminated at a level of just1 microgram per milliliter of the neonicotinoid imidacloprid three times a week.

After two weeks of tainted honey, pesticide-dosed ants moved as briskly as uncontaminated counterparts and showed the same steadiness in navigating a platform without tumbling off.

What changed was combat, which is a common and violent ant event marked by “ants chopping limbs off each other, decapitating each other,” Barbieri says. (He quotes eminent ant biologists Bert Hölldobler and E.O. Wilson, who opined that if ants had nuclear weapons, they’d probably end the world within a week.)

Matchups between 10 ants of each species showed that pesticides had complex effects on combat success. With neither species exposed to pesticides, the southern ants inflicted heavy losses on Argentine ants but did not destroy all of them. Dosing the Argentine ants with pesticides led to their sides’ total destruction against nondosed southern ants. With both sides exposed, the natives lost their edge.

Barbieri is concerned that if real-world fights play out the same way, native southern ants could lose an advantage. Argentine ants can form colonies with tens of thousands of ants, so even one percent survival after pitched battles leaves a lot of survivors.

Aggression between species “is a really hard thing to study in a lab,” cautions ecologist David Holway of the University of California, San Diego. Barbieri agrees that researchers need to perform outdoor tests with natural colonies to see pesticides’ effects in nature.

However those contests turn out, Holway doesn’t expect neonicotinoids to explain how Argentine ants have spread so widely around the world and displaced so many natives. The ants’ takeovers, he says, include remotes spots with little exposure to pesticides.